The present invention relates to a process for oligomerizing ethylene, in particular to 1-hexene, and to the catalytic composition used.
Processes for producing alpha olefins from ethylene generally result in a series of oligomers containing 4 to 30 carbon atoms and even more than 30 carbon atoms, and the olefins are then separated by distillation. The demand for lower olefins, essentially 1-butene, 1-hexene and 1-octene, has been increasing over the past few years; they are used in particular as co-monomers with ethylene in the production of linear low density polyethylene.
Only a few catalysts exist that can selectively lead to the formation of a particular oligomer, as is the case when dimerizing ethylene to butene-1 with a titanium-based catalyst. However, chromium-based catalysts are known to result in the principal formation of 1-hexene, with more or less polyethylene, the proportion of butenes and octenes in the products being very low. (R. M. Manyik, W. E. Walker, T. P. Wilson, J. Catal., 1977, 47, 197 and J. R. Briggs, J. Chem. Soc., Chem. Commun. 1989, 674 and cited references). Catalysts for more or less selective ethylene trimerization have been claimed, for example, in U.S. Pat. Nos. 5,198,563, 5,288,823, and 5,382,738, European patent applications Nos. 0 608 447, 0 611 743 and 0 614 865. Such catalysts are prepared from a chromium salt and a metallic amide, in particular a pyrrole. Other catalysts use an aluminoxane and a chromium complex with a chelating phosphine (U.S. Pat. No. 5,550,305).
French patent application 2 764 524 describes a catalytic composition obtained by mixing at least one chromium compound with at least one aluminum aryloxy compound and at least one hydrocarbylaluminum compound, that has a particular selectivity for the formation of butene-1 and/or 1-hexene by ethylene oligomerization.
It has now been discovered that a catalytic composition obtained by mixing at least one chromium compound with at least one aryloxy compound of an element M selected from the group formed by magnesium, calcium, strontium and barium and with at least one hydrocarbylaluminum compound, has a particular selectivity for the formation of 1-hexene by ethylene oligomerization.
More precisely, said catalytic composition is obtained by mixing:
at least one chromium compound that can comprise one or more identical or different anions selected from the group formed by halides, carboxylates, acetylacetonates, alkoxy and aryloxy anions;
with at least one aryloxy compound of an element M selected from the group formed by magnesium, calcium, strontium and barium, with general formula M(RO)2-nXn, where RO is an aryloxy radical containing 6 to 80 carbon atoms, X is a halogen or a hydrocarbyl radical containing 1 to 30 carbon atoms and n is a whole number that can take values of 0 to 2;
and with at least one aluminum compound selected from hydrocarbylaluminum compounds with general formula AlRxe2x80x2mY3-m, where Rxe2x80x2 is a hydrocarbyl radical containing 1 to 6 carbon atoms, Y is a chlorine or bromine atom and m is a number from 1 to 3 (i.e., tris(hydrocarbyl)-aluminum compounds, chlorinated or brominated hydrocarbylaluminum compounds) and aluminoxanes.
The chromium compound can be a chromium (II) or chromium (III) salt, but also a salt with a different oxidation number that can comprise one or more identical or different anions such as halides, carboxylates, acetylacetonates or alkoxy or aryloxy anions. The chromium compounds preferably used in the invention are chromium (III) complexes as they are more accessible, but a chromium (I) compound or chromium (II) compound may also be suitable.
The chromium compounds selected can advantageously be dissolved in a hydrocarbon medium by complexing with an organic oxygen-containing compound such as an ether, an ester or a compound selected from acetates and ketals (these latter resulting from condensation of an aldehyde or a ketone with a monoalcohol or a polyalcohol) such as 2,2-di(2-ethylhexyloxy)propane.
The aryloxy compound of element M is selected from the group formed by magnesium, calcium, strontium and barium, with (general formula M(RO)2-nXn, where RO is an aryloxy radical containing 6 to 80 carbon atoms. X is a halogen (chlorine or bromine) or a linear or branched hydrocarbyl radical containing 1 to 30 carbon atoms, for example alkyl, cycloalkyl, alkenyl, aryl, aralkyl, substituted aryl or substituted cycloalkyl, preferably a hydrocarbyl residue containing 2 to 10 carbon atoms, and n is a whole number that can take values of 0 to 2.
Preferred aryloxy compounds of element M comprise an aryloxy radical RO with general formula: 
where R1, R2, R3, R4 and R5, which may be identical or different, each represent a hydrogen atom, a halogen atom or a hydrocarbyl radical, for example alkyl, cycloalkyl, alkenyl, aryl, or aralkyl, substituted aryl or cycloalkyl, preferably containing 1 to 16 carbon atoms, more particularly 1 to 10 carbon atoms. Non limiting examples of R1, R2, R3, R4 and R5 are methyl, ethyl, n-propyl, ispropyl, n-butyl, tert-butyl, cyclohexyl, benzyl, phenyl, 2-methylphenyl, 2,6-dimethylphenyl, 2,4,6-trimethlylphenyl, or 2-methyl-2-phenylprop-1-yl residues.
Non-limiting examples of preferred aryloxy radicals that can be cited are: 4-phenylphenoxy, 2-phenylphenoxy, 2,6-diphenylphenoxy, 2,4,6-triphenylphenoxy, 2,3,5,6-tetraphenylphenoxy, 2-tert-butyl-6-phenylphenoxy, 2,4-di-tert-butyl-6-phenylphenoxy, 2,6-diisopropyphenoxy, 2,6-dimethylphenoxy 2,6-di-tert-butylphenoxy, 4-methyl-2,6-di-tert-butylphenoxy, 2,6-dichloro-4-tert-butylphenoxy and 2,6-dibromo-4-tert-butylphenoxy. When the aryloxy compound of element M is selected from aryloxides with general formula M(RO)2, the two aryloxy radicals can be carried by the same molecule, for example the biphenoxy radical, the binaphthoxy radical or the 1,8-naphthalene-dioxy radical, which may or may not be substituted by one or more alkyl, aryl or halogen radicals.
The preparation of the compound M(RO)2-nXn is known in the literature. Any process for preparing this compound is suitable, such as reacting a phenol ROH with a dialkylmetallic element in an organic solvent, for example a hydrocarbon or an ether.
The aluminum compounds used in the invention are selected from hydrocarbylaluminum-tris(hydrocarbyl)aluminum compounds, chlorinated or brominated hydrocarbylaluminum compounds and aluminoxanes. The tris(hydrocarbyl)aluminum compounds and chlorinated or brominated compounds of hydrocarbylaluminum are represented by general formula AlRxe2x80x2mY3-m where Rxe2x80x2 is a hydrocarbyl radical, preferably alkyl, containing 1 to 6 carbon atoms, Y is a chlorine or bromine atom, preferably a chlorine atom, and m is a number from 1 to 3. Non-limiting examples that can be cited are: dichloroethylaluminum, ethylaluminum sesquichloride, chlorodiethyl-aluminum, chlorodiisobutylaluminum, triethylaluminum, tripropylaluminum, triisobutylaluminum and methylaluminoxane. The preferred hydrocarbylaluminum compound is triethylaluminum.
The catalyst components can be brought into contact in a solvent comprising at least one saturated hydrocarbon such as hexane, cyclohexane, heptane, butane or isobutane, at least one unsaturated hydrocarbon such as a mono-olefin or a diolefin containing 4 to 20 carbon atoms, for example, and/or at least one aromatic hydrocarbon such as benzene, toluene, ortho-xylene, mesitylene or ethylbenzene.
The chromium concentration in the catalytic solution can be in the range 1xc3x9710xe2x88x925 to 0.1 mole/l, preferably 5xc3x9710xe2x88x925 to 1xc3x9710xe2x88x922 mole/l. The mole ratio between the aryloxy compound of element M and the chromium compound can be from 1:1 to 30:1, preferably 1:1 to 20:1. The mole ratio between the hydrocarbylaluminum and the chromium compound is in the range 1:1 to 35:1, preferably 1:1 to 15:1.
The order of mixing the three constituents of the catalytic composition is not critical. However, the chromium compound is preferably mixed first with the aryloxy compound of element M, and then the hydrocarbylaluminum compound is added.
The ethylene oligomerization reaction can be carried out at a total pressure of 0.5 to 15 MPa, preferably 1 to 8 MPa, and at a temperature of 20xc2x0 C. to 180xc2x0 C., preferably 50xc2x0 C. to 160xc2x0 C.
In a particular batchwise implementation of the catalytic oligomerization reaction, prepared as described above, a set volume of the catalytic solution, prepared as described above, is introduced into a reactor provided with the usual stirring, heating and cooling means, then pressurized to the desired pressure with ethylene, and the temperature is adjusted to the desired value. The oligomerization reactor is then kept at a constant pressure by introducing ethylene until the total volume of the liquid produced represents, for example, 2 to 50 times the volume of the original catalytic solution introduced. The catalyst is then destroyed by any usual means known to the skilled person, and the reaction products and solvent are extracted and separated out.
For a continuous operation, the following is preferably carried out: the catalytic solution is injected at the same time as the ethylene, into a reactor stirred by conventional mechanical means or by external re-circulation, and kept at the desired temperature. The catalyst components can also be separately injected into the reaction medium, for example the product of the interaction of the chromium compound with the aryloxy compound of element M and the hydrocarbylaluminum compound. Ethylene is introduced via a pressure controlled inlet valve, which keeps the pressure constant. The reaction mixture is withdrawn using a liquid level controlled valve to maintain the liquid level constant. The catalyst is continuously destroyed using any means known to the skilled person, then the reaction products and the solvent are separated, for example by distillation. The non-transformed ethylene can be recycled to the reactor
The entire disclosure of all applications, patents and publications, cited above and below, and of corresponding French application 99/16509, filed Dec. 24, 1999, are hereby incorporated by reference.
The following examples illustrate the invention without limiting its scope.